The long-term purpose of this research is to determine the feasibility of using in vivo EPR to assess radiation does in accidental exposures to radiation. The current research focuses on the feasibility of using low frequency (L-band) EPR spectroscopy for the measurement of long-lived radiation-induced EPR signals in the teeth of living rats and to compare this with similar measurements in isolated human teeth. The radiation-induced EPR signal in teeth has distinctive characteristics based on its shape and power saturation. The use of teeth also has the advantage of providing a site where very good geometry with a sensitive resonator can readily be achieved for entirely non-invasive measurements in vivo. These results suggest that it may be possible to develop techniques and instruments that will allow these measurements eventually to be made non-invasively in humans. We now have completed the initial measurements of radiation-induced signals from the teeth of living rats and isolated human teeth. Initially the signal intensity from rat teeth which received X-ray radiation was quite weak due to its very small size and weight (about 100mg), so while it was detectable with X-band EPR, very high doses were needed to observe it with L-band. We therefore made a new loop resonator specially designed for rat teeth and this increased the sensitivity to a useful level. We also used the new resonator to obtain data from isolated human teeth with a present limit of detection of about 1Gy. Further technical developments should extend this to even lower doses. We conclude that human in vivo EPR dosimetry will be feasible.